Slower, yes, with the 4" coil. You can't cover ground as quickly with a 4" coil. Your SWEEP speed doesn't necessarily have to be slower, but you have to "overlap" your sweeps much more than you would with the bigger coil, so your amount of ground covered will be less, per unit time, if that makes sense. In general, you will also lose some depth with the 4" coil, but ONE -- depth is not everything, and TWO, you won't ALWAYS lose depth. Let me try to explain... The 4" coil can "see between" the trash, due to its smaller "footprint," and thus give you multiple tones for multiple closely-adjacent targets, whereas a larger coil might give you just one tone, representing the "conglomerate" of targets under the coil. This is one benefit of a smaller coil -- you may not find the DEEPEST targets, but you CAN find targets others have missed, trying to go deeper with a larger coil. So, what some guys will do is run over a spot, and use a "deep" (larger) coil to find the deep stuff, and then come back over it with a small coil to find the stuff amongst the junk. Two different ways to recover more targets. Another tactic, if you feel there are definitely good targets in a patch of ground, is to DIG EVERYTHING -- get the junk "out of the way," so you can find the better stuff that was hiding amongst/underneath the junk. Especially with iron, you can get MAJOR masking problems -- meaning that if there is iron near, or on top of, a good target, the iron so "overwhelms" the machine that it can't "see" the good target underneath or adjacent to it. DD coils help with this, but depth in general is severely restrained by iron in the ground. If you can normally see an 8" quarter in your ground, with your machine, but you put some small pieces of iron above that quarter, your "effective" depth may only be a few inches (due to the iron effects). In other words, if the iron pieces are at 2", and the quarter at 8", your machine will often only see the iron (especially with a larger, concentric coil). If the quarter was at 4" under the 2" iron, the machine has a better chance of hinting at the quarter. Point being, in a "trashy" site, max depth is often not attainable ANYWAY, so you don't really "lose" much depth, and might actually GAIN "effective" depth, with the small coil (despite the fact that in a clean-ground situation, the larger coil would see "deeper.") Does that make sense?
Steve